This invention is for a heat engine that:
1. uses a regenerator,
2. uses a displacer,
3. uses a turbine or other gas drive,
4. uses an energy source derived from continuous combustion or solar energy,
5. uses a quasi-constant-pressure process,
6. has very-low emissions,
7. has a very-long, continuous-use service life,
8. operates with an efficiency near the Carnot cycle,
9. operates efficiently when operating at a small fraction of rated output, and
10. belongs to the family of Ericsson cycle engines.
The regenerative gas turbine belongs to the family of Ericsson cycle engines. It is the preferred small gas turbine configuration. This engine has important limitations:
1. back work (work required to drive the compressor) puts a premium compressor component efficiency, i.e., a small drop in component efficiency results in a much larger drop in engine efficiency;
2. small engines have low operating efficiencies;
3. turbine blade life is limited by high temperature metal fatigue and creep failure, significantly adding to operating cost and lowering service life;
4. the engine operates best as a constant output engine, i.e., operating efficiency can be poor at, say, 10% of rated output and thus not useful for many applications;
5. the regenerator requires a high pressure and high temperature gas seal; however, this problem can be overcome by accepting a lower efficiency and using a recuperator in place of the regenerator; and
6. it is costly relative to some engine types.
Gas turbines such as those used on aircraft have gained wide use because they have a low specific weight and are powerful, reliable and durable. However, to achieve good efficiency they require high combustion temperature that results in considerable emissions, and use turbine blades that require costly materials and typically fail due to creep failure or fatigue failure. In addition, they have very poor efficiency when operating at a small fraction of rated power.
Steam power plants operate on the Rankine cycle. There is essentially no back work for this system; however, the efficiency of the Rankine cycle is substantially lower than the Carnot cycle and steam plants consequently are limited to efficiency near 40%. Steam power plants operate efficiently only at a constant output and require a long time to power up.
The spark ignition (SI) engine has a moderate specific weight, cost and efficiency. It has gained universal use as a light-duty automotive engine. The SI engine requires an elaborate emission control system. The SI engine""s high wear rates and service requirements preclude its use for applications requiring long continuous operation. Although better than the gas turbine or steam turbine, it has poor efficiency when operating at a small fraction of rated power.
Compression ignition (diesel) engines have become the premier heavy truck and industrial engine type. They have high emissions, significant wear and require regular maintenance. This engine is not stable at very low engine speeds.
Another regenerative gas cycle engine is the Stirling cycle engine that uses a constant-volume process as opposed to constant-pressure processes. Stirling cycle engine limitations include low volumetric efficiency and high-pressure, pushrod seal wear.
Another regenerative gas cycle engine that uses constant-pressure processes is the Ericsson engine. This engine has not gained significant market acceptance except for small engines and has high wear characteristics.
U.S. Pat. Nos. 2,127,286; 2,175,376; 3,9191,586; 4,133,173; 4,984,432; 5,473,899; 5,590,528 and 5,894,729 have information on several Ericsson cycle engines or related information. However, each of these references suffers from the disadvantages of gas turbines and/or diesel and/or Stirling cycle engines.
Cogeneration units that generate electricity and use rejected heat to provide space heating and to heat water have gained limited acceptance for medium-size commercial and industrial facilities. They are essentially nonexistent for home use. Cogeneration reduces energy consumption and can offer considerable economic advantages to the user. Small cogeneration units such as for a single-family house or small business have not been successful because a heat engine with the necessary requirements has not been available. Such an engine ideally should:
1. operate continuously for at least ten years without the need for servicing;
2. have very low exhaust emissions over the ten-year interval;
3. have a good efficiency at both a very low and high output;
4. have a low manufacturing cost; and
5. ideally, be compatible with solar-based energy augmentation.
Small (5 kW) solar-thermal heat engine driven electric generators have failed to enter the market because the required engine has not been available. Such an engine would be low cost, have a ten-year maintenance-free service life and have a high efficiency.
A large (100 kW) space solar thermal power system has not been used because the required engine has not been available. Such an engine would have a fifteen-year, continuous, maintenance-free service life and have a high efficiency.
Due to cost, coal is the fuel of choice for electric power generation. Coal plants almost exclusively operate on the Rankine cycle and are typically limited to 40% energy conversion efficiencies. They operate as base power plants with a constant output.
Large natural gas electric power plants operate on either (1) the Rankine cycle and are typically limited to 40% efficiencies or (2) a gas turbine cycle and are typically limited to somewhat less than 40% efficiencies or (3) a combined cycle and are typically limited to less than 50% efficiencies.
Engines used with ground transportation systems operate at high combustion temperatures; consequently, they require complex and costly emission control systems, and operate at efficiencies that are much lower than are theoretically possible.
For the foregoing reasons, there is a need for a gas-cycle heat engine with the following capabilities:
1. scales well from 1 kW to 1 GW;
2. has a very long, continuous, maintenance-free service life;
3. has very low emissions without the need for a costly emission control device;
4. operates with an efficiency near the Carnot cycle so that relatively low combustion temperature can be used;
5. has responsive controls;
6. has a version that can be used as part of a home cogeneration unit;
7. has a version that can be used as part of a low-cost, solar-thermal, power system;
8. has a version that can be used as part of a space solar thermal power system;
9. has a version that operates with coal for large power plant use;
10. has a version that can use natural gas very efficiently for large power plants; and
11. has a version that is light and compact so it can operate with ground transportation vehicles, and in addition provide high torque at low speed or ideally a constant power output.
The present invention, a heat engine, satisfies the needs stated in the background. The engine uses continuous combustion, a quasi-constant-pressure process, a thermal compressor (TC) that compresses gas and a drive that transforms the energy in the compressed gas into spinning shaft power. The TC compresses gas directly with heat. Internal pressure is typically high and varied as a means of varying torque output. The TC comprises a means of bringing heat into the engine, a cooler that removes heat, and a TC displacer drive. The engine brings heat in with a heater that uses external combustion or a heater that uses continuous internal combustion or continuous combustion directly in the hot chamber of the TC.
The engine has embodiments that remove (uncouple) heater and/or cooler interior volumes during gas compression. This improves volumetric efficiency, improves fuel use efficiency and enables the engine to scale well to large sizes. The engine cycle closely approximates the efficiency of the Carnot cycle and yields a high efficiency while limiting combustion temperature. Low combustion temperatures allow the engine to operate with very low emissions.
The continuous internal combustion version, which uses air as the working fluid, requires a clean fuel such as natural gas or clean distillates in order to avoid regenerator clogging. It requires a means of compressing air to the internal operating pressure and a means of extracting energy from the products of combustion before discharging them back into the atmosphere. This version can be light, small and powerful.
A version of the engine uses an innovative electromagnetic displacer drive. The displacer is spun and levitated with an integral small clearance seal and gas bearing. A linear electromagnetic motor induces oscillatory translational motion. By inducing a gas-dynamic bounce at the end of each stroke, engine speed increases. This version operates without the need for displacer wear surfaces and is a preferred version for applications, which require a long continuous service life. Another version uses a motor powered crank and pushrod as the displacer drive. A unique method obviates the high-pressure, pushrod-seal, wear problem. Another version of displacer drive uses a slender center rod to support the displacer and a linear electromagnetic drive. Very-low push rod or center rod seal wear occurs by means that equalize the pressure across this seal.
Encapsulation of high-temperature elements in a pressurized chamber and a partial vacuum in the interior of the displacer minimizes tensile stresses in ceramic components and allows the use of low cost ceramics for high temperature components.
The engine uses an innovative staged combustion heater with very low emissions. It does not require a catalytic converter. A compact version of the heater has a monolithic ceramic structure that implements staged combustion.
The engine innovation includes a constant power gas turbine drive. In essence, this drive can vary output torque so that power output remains constant over an operating speed range.
For a small, natural gas home cogeneration system a version of the engine uses an external combustion heater, electromagnetic displacer drive, decoupled cooler, and an impulse turbine that directly drives a constant-speed, 60 Hz, AC generator (costly frequency conversion power electronics are not required). The invention includes a version for use in outer space, as a solar thermal power system that can meet the stringent long continuous, maintenance-free requirements. For an automobile, a version of the engine uses several continuous internal combustion TCs and a constant power turbine drive. For a variable-output, coal power plant a version of the engine uses an external combustion heater, which is similar to a coal boiler. It also uses multiple crank-powered TCs; decoupled heater and cooler; constant speed turbo generator; and a pump system for varying system pressurization as a means for varying turbine torque.